The present invention relates to a solid-state image sensor, and a manufacturing method thereof.
Prior art in the field will be described, referring to the drawing.
FIG. 1 is a view of an example of a circuit of a solid-state image sensor called an amplifying MOS sensor.
In FIG. 1, amplifying transistors 2 for reading a signal from a photodiode 1 and 3xc3x973 unit cells comprising a reset transistor 3 for resetting a signal charge are arranged in a two-dimensional form. In reality, more unit cells than those shown in FIG. 1 are arranged. Horizontal address lines 5 extending in the horizontal direction from a vertical shift resistor 4 are connected to the gates of vertical select transistors, and specify the line for reading signals. Reset lines 6 are connected to the gates of the reset transistors. The sources of the amplifying transistors 2 are connected to vertical signal lines 7, and the one end of each of the lines 7 is connected to a load transistor 8. The other thereof is connected to a horizontal signal line 11 through a horizontal select transistor 10 to be selected by a select pulse supplied from a horizontal shift resistor 9.
In a conventional method for forming a photodiode 1 in a photoelectric conversion section (see FIG. 2), a p-well (p-type layer) 12 is formed on a Si substrate 13, and a resistor 14 is applied onto the Si substrate at other than the portion where the photodiode 1 is to be formed, to carry out patterning. Subsequently, phosphorus 15 (P) is implanted thereto by ion implantation at one time. After that, thermal treatment is conducted to form the photodiode 1. The distribution of impurity concentration is shown in FIG. 3.
FIG. 3 shows the distribution of phosphorus (P) concentration, which is for forming the photoelectric conversion section, the distribution of boron (B) concentration, which is for forming the p-well, and the distribution of net impurity concentration. Such a sort of device has the following disadvantages:
(1) Carriers in a sufficient amount which are generated by photoelectric conversion cannot be stored, since the capacity of the photodiode 1 is small.
(2) Carrier generated in the deep position in the substrate by photoelectric conversion leak into the photodiodes for adjacent pixels so as to result in mixed colors.
(3) The area for collecting the carrier in the photodiode is small and consequently sensitivity is low.
In the case of implanting a great deal of an ion formed by at least twice ion implantation.
Preferred manners of the solid-state image sensor are as follows:
(1) An area of the vicinity of a surface portion of the plurality of photoelectric conversion sections is larger than that of a deep portion of the plurality of photoelectric conversion sections.
(2) The active regions are isolated by an insulator element isolation.
(3) In the first and second ion implantation steps for forming the photoelectric conversion sections sorts of implanted ions are different or energies for implantation are different.
The method of manufacturing the solid-state image sensor according to the invention, wherein the solid-state image sensor comprises a plurality of photoelectric conversion sections, formed within respective isolated active regions on a semiconductor substrate; an image pick-up area wherein unit cells comprising the plurality of photoelectric conversion sections and a signal scanning circuit are arranged in a two-dimensional array form; and signal lines for reading signals from the respective unit cells within the image region, comprises the step of forming the a plurality of photoelectric conversion sections by at least twice ion implantation.
According to the invention, the photodiodes for in the conventional method, ion damage is concentrated into a certain depth (for example, near 0.2 xcexcm) so that dark current increases.
As described above, the conventional MOS-type solid-state image sensor has the disadvantages of small capacity of the photodiode, mixed color, low sensitivity and generation of a dark current in a large amount.
The object of the invention is to provide a solid-state image sensor which has a large photodiode capacity and a high sensitivity and causes no mixed colors and dark current in a small amount; a method of manufacturing the same; and various sorts of devices to which the solid-state image sensor is applied.
To overcome the aforementioned disadvantages, the present invention is as follows:
The solid-state image sensor according to the present invention comprises a semiconductor substrate; a plurality of photoelectric conversion sections formed within respective isolated active regions on the semiconductor substrate; an image area wherein unit cells comprising the plurality of photoelectric conversion sections and a signal scanning circuit are arranged in a two-dimensional array form; and signal lines for reading signals from the respective unit cells within the image pick-up area, wherein the respective photoelectric conversion sections being the photoelectric conversion sections can be formed to be positioned deeply in the deep direction of the substrate, since ion implantation is carried out at least two times. As a result, the area for collecting carriers in the photodiodes is spread in the deep direction of the substrate, thereby improving sensitivity. The degree of color crosstalks can be reduced since carriers leaking into adjacent pixels can be reduced. It is also possible to increase joint capacitance produced at the side wall portion of adjacent pixels. Ion implantation is separately carried out several times, so that areas subjected to ion damage (mainly, near the portion where ions are stopped) can be dispersed. Thus, the ion damage can be easily recovered in a subsequent annealing step, and dark current is reduced.
The following advantages are obtained according to the present invention.
The photodiodes for the photoelectric conversion sections can be formed to be positioned deeply in the deep direction of the substrate. As a result, the area for collecting carriers in the photodiodes is spread in the deep direction of the substrate, thereby improving sensitivity. The degree of color crosstalks can be reduced since carriers leaking into adjacent pixels can be reduced. It is also possible to increase joint capacitance produced at the side wall portion of adjacent pixels. Ion implantation is separately carried out several times, so that areas subjected to ion damage (mainly, near the portion where ions are stopped) can be dispersed. Thus, the ion damage can be easily recovered in a subsequent annealing step, and dark current is reduced.
Application of the present invention to, in particular, a MOS-type solid-state image sensor is very useful; however, application of the invention to any CCD other than the MOS-type solid-state image sensor also provides all advantageous effects except decrease in dark current.
The solid-state imaging system according to the invention comprises an optical system for receiving an optical image from an object to introduce the optical image onto a specified position; an image processor having a sensor for converting the optical image introduced onto the specified position into electric signals corresponding to the quantity of the light of the optical image, for every pixel unit; and a signal processor for processing the outputs from the image processor into a desired format to output the processed signals, wherein the sensor having: a semiconductor substrate; a plurality of photoelectric conversion sections formed within respective isolated active regions on the semiconductor substrate; an image region wherein unit cells comprising the plurality of photoelectric conversion sections and a signal scanning circuit are arranged in a two-dimensional array form; and signal lines for reading signals from the respective unit cells within the image pick-up area; the respective photoelectric conversion sections being formed by at least twice ion implantation.
Preferred manners of this solid-state imaging system is as follows.
(1) The area of the vicinity of the surface portion of the plurality of photoelectric conversion sections is larger than that of the deep portion of the photoelectric conversion sections.
(2) The active regions are isolated by insulator element isolations.
(3) The firs and second ion implantation steps for forming the photoelectric conversion sections sorts of implanted ions are different or energies for implantation are different.
The aforementioned solid-state imaging system according to the invention makes it possible to provide a compact MOS-type solid-state image sensor application system which can realize saving energy and a high quality image.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.